1. Technical Field
The present invention relates to an image forming apparatus for generating a recording timing pulse for driving a recording unit for performing recording with respect to a recording medium on a transporting unit on the basis of an encoder signal for detecting a transporting position of the transporting unit, and a pulse generating method.
2. Related Art
In an image forming apparatus such as a printer, a recording head performs printing on a sheet transported in a transportation direction. In this case, since ink droplets need to be discharged at adequate timings according to the positions of the sheet, a print reference signal is generated in synchronization with a transportation speed of the sheet on the basis of an output signal output from an encoder in synchronization with the transportation speed of the sheet and the discharge timings are controlled on the basis of the print reference signal.
For example, in JP-A-11-245383, a printer (image forming apparatus) using a transportation belt as a sheet transportation unit is disclosed. A mark for detecting the speed and the position is provided on the transportation belt, the mark is read by an encoder, and an ink is discharged on the basis of the encoder signal, thereby printing letters or images on the sheet.
In a printer using the transportation belt as the sheet transportation unit in JP-A-11-245383, under the condition that it is assumed that the movement amount of the belt and the movement amount of the a recording sheet are equal to each other, the movement amount of the recording sheet is detected from the movement amount of the belt and the ink droplets are discharged in every desired pitch. Since the ink droplets are discharged in synchronization with the encoder signal having a pulse having the same interval as a print pitch, high-quality printing for controlling a shift of a striking position can be realized even when the speed of the transportation unit is changed. However, in the method of JP-A-11-245383, since the encoder signal needs to be continuously output at a regular pitch, the following problems occur.
In a case where the circumferential length of the transportation unit (transportation belt) of the recording sheet is not an integral multiple of a print pitch, a discontinuous portion is generated in the output signal of the linear encoder and thus an image deteriorates. In a case where an ink mist or a paper dust is adhered to the linear encoder arranged on the transportation belt or the linear encoder is damaged, a pulse is omitted in the encoder signal. In this case, the image deteriorates.
An apparatus for solving the above-described problems is disclosed in JP-A-2003-280484 (for example, paragraphs [0023] to [0061], FIG. 1, FIG. 9, FIGS. 10 to 22) and JP-A-2005-350195 (for example, [0041] to [0053] and FIGS. 6 to 11).
The printer disclosed in JP-A-2003-280484 controls a motor speed using a linear encoder by a PLL, has a discontinuous detection unit of the linear encoder, and changes a speed/position control unit on the basis of the detected result of the discontinuous detection unit. In the discontinuous portion, an output interval average value of the output signal measured in a continuous portion is used. As countermeasures against a long discontinuous portion, a controlling method using two sensors is described and a problem that the speed of the belt cannot be detected for a long time period is avoided.
In JP-A-2005-350195, two sensors for detecting a mark are provided such that positions to be detected are different from each other and, when a discontinuous portion of the mark such as a joint of a transportation belt or the omission of a pulse due to dust or flaws is detected on the basis of an output signal of one sensor, an output signal used for motor control for constantly controlling a belt transportation speed is switched to the other sensor. In order to suppress a phase difference between the signals when the sensor is switched, a measured period is divided or the same clock is used such that an interpolation processing unit generates a high-resolution signal so as to reduce a signal matching error (phase difference). In JP-A-2003-280484 or JP-A-2005-350195, a method of counting the period of the sensor signal (encoder signal) by a base clock and determining the discontinuous portion (the omission of the pulse signal of the encoder signal) of the mark in a case where the period of the pulse of the sensor signal is equal to or greater than a predetermined threshold value is disclosed.
However, in JP-A-2003-280484, since a method of improving print precision by controlling the motor speed such that the speed of the transportation belt becomes a predetermined speed is employed, a controller for controlling the motor becomes complicated and the control vibrates. In JP-A-2005-350195, since a method of improving print precision by controlling the motor speed such that the speed of the transportation belt becomes a predetermined speed, the control is susceptible to be divergent and thus a control gain needs to be adjusted by an apparatus.